If you still have your old kernel, boot into that and check out the documentation coming with this version of Debian. It states: With version 2.5.3, the Debian mdadm package now requires a mdadm.conf file..

WTF? Was my first reaction. I didn’t install any new mdadm version. But OK, I went with it.

I replaced the old – self-compiled – md-tools with the ones coming with Debian Etch – a quick apt-get install mdadm sufficed. Luckily, debconf automagically created the conf-file.

Tried the reboot into the new kernel – and it worked.

And it was once again proven: Documentation is like sex: When it is good, it is VERY good; and when it’s bad, it’s still better than nothing at all.